Feathers and flesh both contain large amounts of protein. I would start with that. Move your flock to a higher-protein feed and see if that helps.
Also check out your space. Requirements are variable, but the generally accepted minimum is four square feet of space each in the coop (16 square feet = four birds) and ten square feet in the run.
Add some distractions and line-of-sight breaks. My coop contains an upside-down five gallon bucket in the middle, for example (the pullets duck around it and the older hens consider it too much effort to follow them), though that's hardly an efficient deterrent if you already have a problem. One thing you could do is divide your coop lengthwise with an upright piece of plywood that's not touching either wall, so that the hen can run in a circle around it and lose her pursuers. (secure it carefully--no sense having one of your hens smooshed by falling wood)
Invest in a anti-peck product. Some stores sell a tar-like wound care product that tastes really nasty. It may come in the form of an ointment or a paint.
A good rooster stops disputes really well. They break up fights and protect all of their hens. That may not be a viable option for you, but I always like having a rooster around.